Vapor generators



Feb. 5, 1963 A. BELL VAPOR GENERATORS Fil /&

Jan, 51, 196? (NILE/V731? /4 BY Mid/M M United States Patent This invention relates to steam generators in which primary steam and water separators are located a substantial distance above the Working water level in a drum. The terms water and steam are used in this specification to denote any other suitable liquid an its vapor.

A typical unit to which the invention may be applied comprises an upright cylindrical vessel having a tube bundle arranged in its lower part and a steam and water separator in its upper part substantially above the operating water level. The steam and water separator receives steam generated below the water level by the passage of a heating fluid through the tube bundle, as part of a steam and water mixture. The steam and Water mixture is conveyed from the body of water by way of a riser.

In such a unit there is a considerable risk that when the steam demand decreases, a point may be reached at which the diiference in static water head between the inside and the outside of the riser may be insufiicient to maintain a positive circulation of steam and water through a riser and the. separator. A free surface will then be efiectively formed in the riser, above the water level but below the level of the separator. Steam will be liberated at this free surface and the only water available to flow down the downcomer provided between the steam space in the upper part of the vessel and the lower end of the tube bundle, will be feed water supplied to the unit. This loss of circulation has several disadvantages. For one thing the temperature in the downcomer may be reduced well below the saturation value by the injection of cool feed water, giving rise to objectionable thermal stresses. Also the momentum of the circulating water will be low and this may affect the ability of the unit to respond to a sudden demand for steam.

In accordance with the invention this risk is eliminated by incorporating a separator bypass hole or plurality of bypass holes in the riser wall at substantially the operating water level. This separator bypass hole or holes must have a cross-sectional area substantially smaller than the flow area through the separators so that there shall be no undesirable affect on the steam and water separation or drum turbulence under full load. The cross-sectional area should on the other hand be sufficient to give a useful boost to the circulation under low load. The flow of steam through the hole or holes may be ducted or directed so as to minimize disturbance of drum conditions.

In order that the invention may be understood, its application to a vertical steam generator will be described with reference to the accompanying drawings, in which:

FIGURE 1 illustrates a typical steam generator embodying the concepts of the invention;

FIGURE 2 is a partial sectional view of a modified form of the invention.

The generator comprises an upright cylindrical vessel 12, with dome ends 14 and 16, having a diameter approximately of its height. A tube sheet 18 is provided in the lower end of the vessel dividing the vessel into upper and lower areas, the lower area containing inlet and outlet connections 20 and 22 for the flow rators.

Patented Feb. 5, 1963 of hot fluid into and out of the vessel. The upper area is divided by a cylindrical shell or shroud 24 into a downcomer passageway 26 and a vapor generation chamber 28. A bundle of inverted U-tubes 30 is located immediately below the designed operating water level approximately /a of the way up the vessel, the ends of the tubes being fastened to the tube sheet and in communication with the hot fluid inlet and outlet connections 20 and 22 for circulation of the heating fluid.

The bottom edge of thecylindrical shroud engages the tube sheet and is provided with apertures 32 between the lower end of the downcomer passageway and the lower part of the vapor generation chamber by which feed-water entering the vessel immediately below the water level through line 33 and water separated from steam generated within the shroud may be returned to the lower end of the tube bundle in continuous circulation.

The upper end of the shroud is connected, by a section 34 of reduced diameter closed at its top by a plate 36 defining a central riser 38, to centrifugal steam and water separators 40 arranged in the upper part of the vessel substantially abovethe operating Water level, the steam and water separators being cylindrical vessels spaced annularly at intervals around the riser and of conventional design. In operation. a mixture of steam and Water. passes up the riser into the steam and water sepa- The separated steam leaves the separators by way of outlets 42 at their upper ends while the Water is returned to the body of water in the lower part of the vessel outside the shroud by-way of drains 44 from the separators.

In the normal way the upper. end of the shroud would be closed except for the central riser. In accordance with the invention, in order to avoid the risk of a failure of circulation at low load, the riser is apertured at the normal operating water level by a plurality of annular holes 48, the diameter of which is determined by the requirement that it shall be sufiiciently large to give a substantial boost to the steam and water circulation under low load while not being so large as to interfere with separation under normal loads.

An annular bafiie 50 is provided around the outer edges of the holes having a portion 46 extending across the direct path of liquid and vapor through the holes so that a steam and water mixture issuing from the holes is inwardly directed, thereby avoiding an adverse turbulence and disturbance of drum conditions, and the entrainment of steam in the downcomer passageways.

A modified form of the invention is illustrated in FIG. 2 comprising a plurality of square holes 52 with individual deflector boxes 54 extending across but spaced from the inner sides of the holes, the bottom legs 56 of the boxes extending through the holes into the downcomer passageway and providing a surface deflecting the vapor and liquid flow away from the passageway. Each box 54 cooperates with the riser wall to form a tunnellike passageway leading to the hole and a tortuous path for the vapor and liquid flow. The arrangement reduces the adverse effect on separation at full load conditions, in that the boxes 54 seem to otter a substantial restriction to the flow at this load.

In a test conducted on a conventional generator having a full load circulating ratio of about 9.8 and a downcomer velocity of 3.1 feet per second, it was found that the circulation practically ceased at about 40% full load and the downcomer velocity was no longer measurable. A bypass arrangement similar to that illustrated in FIG. 2 was provided comprising one inch square holes placed beneath each separator, in this instance, nine (9) approximately equally spaced around the base of the riser at the normal operating water level. Each hole was provided with a deflector box fitted around it, as shown, and the downcomer velocity improved to .5 feet per second with a circulation ratio increase to 3.75. The arrangement resulted in no substantial adverse effect on full load carryover or downcomer entrainment.

Many modifications of the invention will be apparent to those skilled in the art, and the invention should be limited in scope only as defined inthe following claims.

What is claimed is:

1. A vapor generator comprising an upright pressure vessel having an upper vapor space and a lower liquid space, a vapor outlet from said vapor space, a shroud within said vessel defining a vapor generation chamber but spaced from said vessel to define with the vessel an annular downcomer passageway, a riser at the upper end of said vapor generation chamber, vapor-liquid separation means in communication with said riser by which vapor flows to said vapor space and separated liquid flows to said downcomer passageway, and means by which said riser is apertured at approximately the normal operating water level of said vessel permitting a vaporliquid mixture to flow directly from said vapor generation chamber to said downcomer passageway, the size of the aperture means being sutficiently large relative to the size of said vapor-liquid separation means to give a substantial boost to the vapor and liquid circulation under low load conditions, but sufiiciently small so as to not adversely afi'ect separation under normal loads.

2. A generator according to claim 1 wherein the aperture means comprises a plurality of spaced holes positioned near but below the normal liquid level in said vessel.

3. A generator according to claim 2 including means to deflect the vapor-liquid fiow through said aperture means inwardlyaway from the outer wall of said pressure vessel to avoid undue passage of steam and undue disturbance within the drum caused by the vapor and liquid issuing through the aperture means.

4. A generator according to claim 3 including plate means covering but spaced from the inner sides of said holes to restrict the flow of the vapor-liquid mixture at full load conditions.

5. A vapor generator comprising an upright pressure vessel having an upper vapor space and a lower liquid space, a vapor outlet from said vapor space, a shroud within said vessel defining a vapor generation chamber but spaced from said vessel to define with the vessel an annular downcomer passageway, a riser at the upper end of said vapor generation chamber, a plurality of vaporliquid separation means spaced around said riser and in communication therewith by which vapor fiows to said vapor space and separated liquid flows to said downcomer passageway, and by-pass means by which said riser is apertured at approximately the normal operating water level of said vessel permitting a vapor-liquid mixture to flow directly from said vapor generation chamber to said downcomer passageway, said by-pass means comprising a plurality of holes spaced at intervals around said riser and beneath each of said vapor-liquid separation means, each of said holes having means by which the vapor and liquid flow is deflected inwardly away from the outer wall of said pressure vessel.

References Cited in the file of this patent UNITED STATES PATENTS 2,862,479 Blaser et al. Dec. 2, 1958 FOREIGN PATENTS 952,984 Germany Nov. 22, 1956 

1. A VAPOR GENERATOR COMPRISING AN UPRIGHT PRESSURE VESSEL HAVING AN UPPER VAPOR SPACE AND A LOWER LIQUID SPACE, A VAPOR OUTLET FROM SAID VAPOR SPACE, A SHROUD WITHIN SAID VESSEL DEFINING A VAPOR GENERATION CHAMBER BUT SPACED FROM SAID VESSEL TO DEFINE WITH THE VESSEL AN ANNULAR DOWNCOMER PASSAGEWAY, A RISER AT THE UPPER END OF SAID VAPOR GENERATION CHAMBER, VAPOR-LIQUID SEPARATION MEANS IN COMMUNICATION WITH SAID RISER BY WHICH VAPOR FLOWS TO SAID VAPOR SPACE AND SEPARATED LIQUID FLOWS TO SAID DOWNCOMER PASSAGEWAY, AND MEANS BY WHICH SAID RISER IS APERTURED AT APPROXIMATELY THE NORMAL OPERATING WATER LEVEL OF SAID VESSEL PERMITTING A VAPORLIQUID MIXTURE TO FLOW DIRECTLY FROM SAID VAPOR GENERATION CHAMBER TO SAID DOWNCOMER PASSAGEWAY, THE SIZE OF THE APERTURE MEANS BEING SUFFICIENTLY LARGE RELATIVE TO THE SIZE OF SAID VAPOR-LIQUID SEPARATION MEANS TO GIVE A SUBSTANTIAL BOOST TO THE VAPOR AND LIQUID CIRCULATION UNDER 